Spontaneous Nanobelt Formation by Self-Assembly of β-Benzyl GABA

Article abstract of DOI:10.1002/asia.201900363

We present the formation of a nanobelt by self-assembly of β-benzyl GABA (γ-aminobutyric acid). This simple γ-amino acid building block self-assembled to form a well-defined nanobelt in chloroform. The nanobelt showed distinct optical properties due to π–π interactions. This new-generation self-assembled single amino acid may serve as a template for functional nanomaterials.

Full text of DOI:10.1002/asia.201900363

CHEMISTRY
AN ASIAN JOURNAL
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Accepted Article
Title: Spontaneous Nanobelt Formation by Self-Assembly of β-benzyl
GABA
Authors: Hee-Seung Lee, Aram Jeon, Jintaek Gong, Jun Kyun Oh,
Sunbum Kwon, Wonchul Lee, Sang Ouk Kim, and Sung June
Cho
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To be cited as: Chem. Asian J. 10.1002/asia.201900363
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Spontaneous Nanobelt Formation by Self-Assembly of β-benzyl
GABA
Aram Jeon,†‡[a] Jintaek Gong,†[a] Jun Kyun Oh, Sunbum Kwon, Wonchul Lee, Sang Ouk Kim,
[b]
[c]
[a]
[d]
[
e]
[a]
Sung June Cho, and Hee-Seung Lee*
Abstract: We present the formation of a nanobelt by self-assembly of
β-benzyl GABA (γ-aminobutyric acid). This simple γ-amino acid
building block self-assembled to form a well-defined nanobelt in
chloroform solvent. The nanobelt showed distinct optical properties
due to π–π interactions. This new-generation self-assembled single
amino acid may serve as a template for functional nanomaterials.
reported as they readily self-assemble into well-defined
superstructures. This is realized in the bilayer structure, which is
the representative structure formed spontaneously via self-
assembly of amphipathic peptides [
3d, 4b, 4d, 5]
However, the inherent structural complexities of peptides,
such as backbone flexibility and ill-defined secondary structures,
remain crucial hurdles in their effective application as building
block molecules. This has led to increasing demand for the
development of simpler building blocks with low molecular weight
that are capable of producing well-ordered supramolecular
structures with manipulable assembly mechanisms. In particular,
single amino acids are suitable for meeting this demand because
they allow simple and cost-effective experimental procedures for
self-assembly experiments.[ This attempting to implement a
complex self-assembled system using a simpler building block is
a common issue in bottom-up approach, and not limited to peptide
self-assembly studies,[7] indeed. In this vein, some sets of
The majority of scientific research on the self-assembly of
amyloid β (Aβ) has been conducted from a pathological point of
view, because the self-assembly of Aβ is closely related to
[
1]
Alzheimer's disease. In particular, numerous studies have been
devoted to inhibiting the formation of amyloid fibrils in vivo and in
vitro, in order to overcome this chronic neurodegenerative
disease.[2] Interestingly, this self-assembly phenomenon has also
evoked interest in an entirely different field of research, as
chemists have identified useful structural properties of amyloid
fibrils. The formation of fibrils under conditions found inside living
organisms has inspired chemists to produce well-defined nano-
6]
molecules have been reported previously to form
a
supramolecular structure of a single amino acid; however they
require an additional protecting group or a specific condition, such
as evaporation of the solution on a surface.[8]
/
micro-sized functional materials with amyloid-like peptides.[3]
The building block commonly used in these studies is an
amphipathic peptide with alternating hydrophobic and hydrophilic
residues, usually represented as (XZXZ)
.[4] The well-known β-
strand forming (FKFE) peptides are chains in which hydrophobic
phenyl) and hydrophilic (ammonium and carboxylate) functional
groups are positioned in an alternating fashion. Thus, various
n
n
(
n
functional materials using the (FKFE) peptide have been
‡
[
a]
Dr. A. Jeon , Dr. J. Gong, Dr. W. Lee, Prof. H.-S. Lee
Department of Chemistry
Center for Multiscale Chiral Architectures(CMCA)
KAIST
Scheme 1. Molecular design of the building block for self-assembly, inspired by
the typical bilayer structure of (FKFE)
side chains of phenylalanine, lysine and glutamic acid, respectively. The
backbone structure of (FKFE) peptide is shown as the bold solid-line. β-benzyl
GABA (β-benzyl γ-aminobutyric acid, γ-Phe) has all key functional groups
similar to (FKFE) peptide (Ph, NH , and COOH).
n
peptide. Here, F, K and E represent the
2
91 Daehak-ro, Yuseong-gu, Daejeon 34141 (Republic of Korea)
n
‡
Current address ; Organic Materials Lab., Materials Center, SAIT,
SEC, 130 Samsung-ro, Yeongtong-gu, Suwon-si, Gyeonggi-do
n
2
16678 (Republic of Korea)
E-mail: hee-seung_lee@kaist.ac.kr
Prof. J. K. Oh
Department of Polymer Science and Engineering
Dankook University
Herein, we introduce a simple γ-amino acid building block, β-
benzyl GABA (β-benzyl γ-aminobutyric acid, γ-Phe), that is
designed to mimic the fragment of the bilayer structure of
[
b]
1
52 Jukjeon-ro, Suji-gu, Yongin-si, Gyeonggi-do 16890 (Republic of
n
amphipathic (FKFE) peptides (Scheme 1). As shown in the
Korea)
scheme, it is proposed that functional groups present in glutamic
acid and lysine could be extracted as carboxylic acid and amino
groups respectively, as present in the backbone of γ-amino acid.
In addition, by maintaining the benzyl group, all functional groups
of the tri-peptide FKE can be installed in a single amino acid. This
would be the smallest building block that has all the advantages
of the amphipathic. Subsequently, it was found that this building
block forms well-defined nanobelts with photoluminescence
properties, through a spontaneous self-assembly process.
[
[
[
c]
d]
e]
Prof. S. Kwon
Department of Chemistry
Chung-Ang University
8
4 Heukseok-ro, Dongjak-gu, Seoul 06974 (Republic of Korea)
Prof. S. O. Kim
Department of Materials Science and Engineering
KAIST
2
91 Daehak-ro, Yuseong-gu, Daejeon 34141 (Republic of Korea)
Prof. S. J. Cho
Department of Applied Chemical Engineering
Chonnam National University
7
7 Yongbong-ro, Buk-gu, Gwangju 61186 (Republic of Korea)
This γ-amino acid, (S)-γ-Phe, was synthesized in an
enantiopure form by following the procedure given in an earlier
study (see ESI for the detailed procedure).[ When a solution of
(S)-γ-Phe (10 μL, 8 g L−1 in ethanol) was added to chloroform (1
9]
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the document.
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mL), it self-assembled spontaneously to form well-defined
nanobelts within a few minutes of sonication (Figure 1), whereas
natural α-Phe did not undergo self-assembly under the same
conditions. Scanning electron microscopy (SEM) images of the
filtered material shows fairly homogeneous nanobelt morphology
with the length ranging from tens to hundreds of micrometers
axis along the b-axis (see ESI for detailed information) during the
final Rietveld refinement step. The March-Dollase preferred
orientation model[11] was adopted for the [100] and [010] axes to
approximate the morphological characteristic of the raw powder,
which consisted of both plate-like and needle-like structures
(Figure S1). The refinement converged to give R
p
= 8.16 %, Rwp
(
Figure 1a–b). The width of the nanobelts is in the range of 0.1–1
= 10.69 %, and Rexp = 4.89 %, and the obtained March-Dollase
[
11]
μm and the thickness of the nanobelts is about half the width, as
shown in the cross-sectional SEM image in Figure 1c. When self-
assembly takes place in presence of phosphotungstic acid (used
for negative staining in TEM analysis), identical nanobelts are
formed. Several parallel dark lines are clearly seen along the axis
of the nanobelt in the TEM image (Figure 1d). The occurrence of
these dark lines, with a period of 5–10 nm, is probably due to the
adhesion of the staining reagent along the hydrophilic region of
the nanobelts.
preferred orientation coefficients (PMD
)
were found to be 0.639
and 1.517 along the [100] and [010] directions, respectively. The
diffractometer was used in the Bragg-Brentano geometry, and the
characteristic morphology of the raw powder was presumed to
have produced a strong texture on the sample. The refined
structure (Figure 2c) revealed a unique network in the ab plane,
consisting of two orthogonal hydrogen bonds along the a and b
axes. No typical π–π interactions were found inside this structure.
Figure 2. a) PXRD patterns of γ-Phe raw powder and nanobelt, b) SEM image
of the self-assembled structures of γ-Phe. They have a preferred orientation
along the [010] direction. c) The refined structure of γ-Phe monomer in the raw
powder form.
Figure 1. a, b) SEM images and c) a cross-sectional SEM image, of the self-
assembled structures of γ-Phe. d) High-resolution TEM image of co-assembled
structures of γ-Phe with staining reagent (phosphotungstic acid); parallel dark
lines are clearly seen in the image (yellow arrow).
Based on the morphological similarity with the needle-like
structure of the raw powder, and by the comparison of PXRD
patterns and PMD values, it was predicted that the longitudinal
direction of the nanobelts would be parallel to the [010] direction
To investigate the molecular packing structure of γ-Phe
monomers in the self-assembled nanobelts, powder X-ray
diffraction (PXRD) analysis of the nanobelts and the raw powder
was carried out. Both samples showed considerably strong and
distinct peaks, indicating high crystallinity. Unfortunately, the
diffraction pattern of the nanobelts could not be indexed with a
single-phase unit cell. However, it should be noted that the
diffraction pattern of the nanobelts has all diffraction peaks
corresponding to the raw powder. This indicates that the
molecular packing structure of the nanobelts is either a supercell
structure of the raw powder, or that the raw powder is present as
an impurity phase in the sample. In other words, the nanobelts
are expected to have a packing structure identical to that of the
raw powder, with small differences in the conformation such as in
the orientation of aromatic rings or the existence of disorders.
Thus, we approximated the molecular packing structure of the
nanobelts from the raw γ-Phe monomer powder (Figure S1). For
the raw powder, a monoclinic unit cell with a = 7.966 Å, b = 6.400
Å, c = 10.4242 Å, and β ≈ 90° was found to be a probable
candidate with a moderately high figure of merit value (F.O.M =
(
Figure 2, S1). Nonetheless, the cause of the formation of a well-
defined structure for the nanobelts during the self-assembly
process, in contrast with the raw powder, remained obscure. We
hypothesized that there are additional intermolecular and π–π
interactions in the nanobelt structure that explain this difference
in behavior. These interactions can be realized in the nanobelt by
the possible aggregation of two phenyl rings in the unit cell in a
head-to-tail arrangement to form a J-dimer.
To confirm the PXRD results, absorption and
photoluminescence (PL) measurements were carried out. The
absorption spectrum of the nanobelt structure, in which phenyl
rings are expected to be well-aligned, is significantly different from
that of the raw powder (Figure 3a). While a typical absorption
peak (located at 257 nm) due to the aromatic ring of γ-Phe residue
is observed for the raw powder (red line), a broad absorption
feature centered below 250 nm with tails extending beyond our
measurement range is observed for the nanobelts (black line).
This demonstrates a clear tendency towards aromatic stacking
along the longitudinal axis of an ordered 2D-structure in the
nanobelts.
[
10]
5
5.7 ). This structure was further refined and its symmetry
lowered to have a triclinic unit cell containing a pseudo-2 screw
1
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In addition, the nanobelts display different PL properties from
that of the raw powder (Figure 3b). Upon excitation at 260 nm (the
excitation wavelength of phenyl group), the characteristic peak
corresponding to the phenyl ring is found at 280 nm, whereas an
additional broad peak appears in the range of 350–450 nm only
for the nanobelts. To further investigate the nature of PL in the
nanobelts, we obtained the photoluminescence excitation (PLE)
spectra at two emission wavelengths of 280 nm and 395 nm (blue
and black lines in Figure 3c, respectively). The first excitation
spectrum (blue line) exhibits a sharp peak centered at about 260
nm, which corresponds to the aforementioned inherent property
of aromatic rings. On the other hand, the second spectrum (black
line) reveals that the center of the emission peak for a 395 nm
excitation is located at a wavelength of 330 nm. Thus, when the
excitation wavelength is 260 nm, the PL spectrum of the
nanobelts has a relatively lower intensity for the 395 nm peak
compared to the 280 nm peak, as seen in Figure 3b. Figure 3d
Additional control experiments proved that the existence of the
phenyl group for π–π interaction was necessary to form the well-
defined nanostructures, such as nanobelt shape in this case. This
is clearly supported by the self-assembly studies using other γ-
amino acids (β-isobutyl-GABA (pregabalin) and gabapentin) with
aliphatic side chains. None of the cases provided well-defined
structures, as shown in Figure S8.
In conclusion, we present an interesting example for
producing well-defined nanomaterials with cost-efficient building
blocks. The β-benzyl GABA used in this study revealed a
characteristic of efficient self-assembly into a nanobelt structure
containing fibrillar bilayers similar to that of the amphipathic
n
(FKFE) sequence. Additionally, the single amino acid self-
assembles spontaneously into well-defined nanobelts with PL
properties due to the π-conjugated system. We have thus
presented an example illustrating how to design a bio-inspired,
molecular building block for effective and spontaneous
association by introducing the requisite functional groups in a
proper geometry and conformation. It is expected that the present
results will lend new insight into the design principle of molecular
building blocks for functional complex nanostructures by self-
assembly from a cost-efficiency point of view. In addition, this
strategy of using amyloid-inspired peptides for nano-engineering
can be applied to multiple fields such as biomedicine, tissue
engineering, wound healing, and drug delivery.
shows
a comparison of PL intensity with the excitation
wavelengths 330 nm (black line) and 260 nm (blue line). The peak
intensity at 395 nm is three times stronger than at 280 nm. These
features of the PL spectra can be observed in polymer
compounds containing aromatic groups due to the transition of
the π* excited state to the π state. The broad excitation and
emission bands may indicate the existence of several
“conjugation lengths” (the degree of π-stacking of γ-Phe) with
various HOMO–LUMO energy band gaps in the nanobelts.
Acknowledgements
†
A. Jeon and J. Gong contributed equally to this work. We
thank Mr. Joo Min Kim and Ms. Danim Lim (KAIST) for assistance
in preparing this manuscript. This research was supported by
National Research Foundation (NRF) of Korea grant funded by
the Ministry of Science and ICT (2016R1A2A1A05005509,
2
018R1A5A1025208).
Keywords: Self-assembly • Nanostructure • Amino acid •
Photoluminescence
[
1]
a) O. S. Makin, L. C. Serpell, FEBS J. 2005, 272, 5950–5961; b) F. Chiti,
C. M. Dobson, Annu. Rev. Biochem. 2006, 75, 333–366; c) M. Fändrich,
Cell. Mol. Life Sci. 2007, 64, 2066–2078; d) L.-M. Yan, A. Velkova, M.
Tatarek-Nossol, E. Andreetto, A. Kapurniotu, Angew. Chem. Int. Ed.
2
007, 46, 1246–1252; e) J. Habchi, S. Chia, C. Galvagnion, T. C. T.
Figure 3. a) Absorption spectra of γ-Phe nanobelt (black line) and γ-Phe raw
powder (red line). b) PL spectrum (excitation wavelength: 260 nm) of γ-Phe
nanobelt (black line) and γ-Phe raw powder (red line). There are two distinct
peaks in for the nanobelt: a sharp peak located at 280 nm (blue arrow) and a
broad peak located at 395 nm (black arrow). c) PLE spectrum of γ-Phe nanobelt
obtained while detecting the emission at two wavelengths: 395 nm (black line)
and 280 nm (blue line). d) PL spectrum of γ-Phe nanobelt at two excitation
wavelengths: 330 nm (black line) and 260 nm (blue line).
Michaels, M. M. J. Bellaiche, F. S. Ruggeri, M. Sanguanini, I. Idini, J. R.
Kumita, E. Sparr, S. Linse, C. M. Dobson, T. P. J. Knowles, M.
Vendruscolo, Nat. Chem. 2018, 10, 673–683.
[
2]
a) N. Gao, H. Sun, K. Dong, J. Ren, X. Qu, Chem. Eur. J. 2014, 21, 829–
8
35; b) S. I. A. Cohen, P. Arosio, J. Presto, F. R. Kurudenkandy, H.
Biverstål, L. Dolfe, C. Dunning, X. Yang, B. Frohm, M. Vendruscolo, J.
Johansson, C. M. Dobson, A. Fisahn, T. P. J. Knowles, S. Linse, Nat.
Struct. Mol. Biol. 2015, 22, 207–213; c) A. J. Doig, P. Derreumaux, Curr.
Opin. Struct. Biol. 2015, 30, 50–56; d) N. Xiong, X.-Y. Dong, J. Zheng,
F.-F. Liu, Y. Sun, ACS Appl. Mater. Interfaces 2015, 7, 5650–5662; e)
W.-J. Du, J.-J. Guo, M.-T. Gao, S.-Q. Hu, X.-Y. Dong, Y.-F. Han, F.-F.
Liu, S. Jiang, Y. Sun, Sci. Rep. 2015, 5, 7992; f) A. Francioso, P. Punzi,
A. Boffi, C. Lori, S. Martire, C. Giordano, M. D’Erme, L. Mosca, Bioorg.
Med. Chem. 2015, 23, 1671–1683; g) T. Su, T. Zhang, S. Xie, J. Yan, Y.
Wu, X. Li, L. Huang, H.-B. Luo, Sci. Rep. 2016, 6, 21826; h) X.-L. Bu, P.
P. N. Rao, Y.-J. Wang, Mol. Neurobiol. 2016, 53, 3565–3575.
All these experimental results lead us to the conclusion that
the conformational preorganization of γ-Phe through
intramolecular hydrogen bonding plays an important role in
accelerating and guiding the self-association in the solution phase.
This is achieved by considerably reducing the entropic cost that
must be overcome to form the preferred linear ordering. The π–π
interactions also play a crucial role and are well-balanced with
hydrogen bonding in association with the building blocks.
[3]
a) G. Wei, Z. Su, N. P. Reynolds, P. Arosio, I. W. Hamley, E. Gazit, R.
Mezzenga, Chem. Soc. Rev. 2017, 46, 4661–4708; b) S. Das, R. S.
For internal use, please do not delete. Submitted_Manuscript
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Chemistry - An Asian Journal
10.1002/asia.201900363
COMMUNICATION
Jacob, K. Patel, N. Singh, S. K. Maji, Biomacromolecules 2018, 19,
[7]
a) M. Yamanaka, J. Incl. Phenom. Macrocycl. Chem. 2013, 77, 33–48;
b) A. J. Kleinsmann, N. M. Weckenmann, B. J. Nachtsheim, Chem. Eur.
J. 2014, 20, 9753–9761; c) B. J. Cafferty, I. Gállego, M. C. Chen, K. I.
Farley, R. Eritja, N. V. Hud, J. Am. Chem. Soc. 2013, 135, 2447–2450;
d) A. Jesorka, A. R. Holzwarth, A. Eichhöfer, C. M. Reddy, Y. Kinoshita,
H. Tamiaki, M. Katterle, J.-V. Naubron, T. S. Balaban, Photochem.
Photobiol. Sci. 2012, 11, 1069–1080; e) G. Ragazzon, M. Baroncini, S.
Silvi, M. Venturi, A. Credi, Beilstein J. Nanotechnol. 2015, 6, 2096–2104.
a) P. Koley, A. Pramanik, Adv. Funct. Mater. 2011, 21, 4126–4136; b) P.
Koley, A. Pramanik, J. Mater. Sci. 2014, 49, 2000–2012; c) C. Ménard-
Moyon , V. Venkatesh, K. V. Krishna, F. Bonachera, S. Verma, A. Bianco,
Chem. Eur. J. 2015, 21, 11681–11686; d) P. Chakraborty, E. Gazit,
ChemNanoMat 2018, 4, 730–740.
1826–1839; c) I. Cherny, E. Gazit, Angew. Chem. Int. Ed. 2008, 47,
4062–4069; d) T. P. J. Knowles, R. Mezzenga, Adv. Mater. 2016, 28,
6546–6561.
[
4]
a) C. J. Bowerman, B. L. Nilsson, Biopolymers 2012, 98, 169–184; b) R.
J. Betush, J. M. Urban, B. L. Nilsson, Pept. Sci. 2018, 110, e23099; c) J.
Mangelschots, M. Bibian, J. Gardiner, L. Waddington, Y. Van Wanseele,
A. Van Eeckhaut, M. M. D. Acevedo, B. Van Mele, A. Madder, R.
Hoogenboom, S. Ballet, Biomacromolecules 2016, 17, 437–445; d) D. Li,
E. M. Jones, M. R. Sawaya, H. Furukawa, F. Luo, M. Ivanova, S. A.
Sievers, W. Wang, O. M. Yaghi, C. Liu, D. S. Eisenberg, J. Am. Chem.
Soc. 2014, 136, 18044–18051.
[8]
[9]
[
5]
6]
a) R. J. Swanekamp, J. T. M. DiMaio, C. J. Bowerman, B. L. Nilsson, J.
Am. Chem. Soc. 2012, 134, 5556–5559; b) K. Matsuura, Chem.
Commun. 2018, 54, 8944–8959.
T. Ok, A. Jeon, J. Lee, J. H. Lim, C. S. Hong, H.-S. Lee, J. Org. Chem.
2007, 72, 7390–7393.
[
a) P. W. J. M. Frederix, G. G. Scott, Y. M. Abul-Haija, D. Kalafatovic, C.
G. Pappas, N. Javid, N. T. Hunt, R. V. Ulijn, T. Tuttle, Nat. Chem. 2014,
[10] P. M. de Wolff, J. Appl. Crystallogr. 1972, 5, 243.
[11] W. A. Dollase, J. Appl. Crystallogr. 1986, 19, 267–272.
7, 30; b) K. Tao, A. Levin, L. Adler-Abramovich, E. Gazit, Chem. Soc.
Rev. 2016, 45, 3935–3953; c) C. Guo, Z. A. Arnon, R. Qi, Q. Zhang, L.
Adler-Abramovich, E. Gazit, G. Wei, ACS Nano 2016, 10, 8316–8324.
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Entry for the Table of Contents (Please choose one layout)
Layout 2:
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Aram Jeon, Jintaek Gong, Jun Kyun Oh,
Sunbum Kwon, Wonchul Lee, Sang Ouk
Kim, Sung June Cho, and Hee-Seung
Lee*
(
(Insert TOC Graphic here))
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Spontaneous Nanobelt Formation by
Self-Assembly of β-benzyl GABA
Self-assembly of a Single Amino Acid : The β-benzyl GABA (β-benzyl γ-
aminobutyric acid) was designed as a simple single amino acid building block to
n
mimic the fragment of the bilayer structure of amphipathic (FKFE) peptides, which
is commonly used in amyloid-inspired peptide self-assembly studies. This γ-amino
acid self-assembled spontaneously to form a well-defined nanobelt, showed notable
optical properties based on π–π interaction.
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